1. Field of the Invention
This invention relates to a process for phosphating metal surfaces and, more particularly, to a process for the production of manganese- and nickel-containing zinc phosphate coatings on steel, zinc, and/or alloys thereof. These nickel- and manganese-containing zinc phosphate coatings are applied by spray coating, spray/dip coating and dip coating with aqueous solutions.
Processes for phosphating surfaces of iron, steel, zinc and alloys thereof and also aluminum have long been known (Ullmanns Encyklopadie der technischen Chemie, 4th Edition, Vol. 15, pages 686 and 687). The object of phosphating the surfaces mentioned is to increase the adhesive strength of paint films and to improve protection against corrosion.
Acidic zinc and alkali metal phosphate solutions have acquired the greatest significance for phosphating processes. Zinc phosphating baths may contain, for example, monozinc phosphate, free phosphoric acid, zinc nitrate and oxidizing agents as their principal components. The pH value of such solutions is typically in the range from 2.8 to 3.4. The process essentially comprises two reactions, namely: the pickling reaction and the formation of a zinc phosphate coating on the surface to be phosphated.
Manganese-modified zinc phosphate coatings as a primer for modern paints are known from W.A. Roland and K.-H. Gottwald, Metalloberflache, Vol. 42, 1988/6. According to this publication, the use of manganese ions in addition to zinc and nickel ions in low-zinc phosphating processes demonstrably improves protection against corrosion, particularly where surface-treated fine metal sheets are used. The incorporation of manganese in the zinc phosphate coatings leads to relatively small and relatively compact crystals showing increased stability to alkalis. At the same time, the operating range of phosphating baths is increased; even aluminum in conjunction with steel and galvanized steel can be phosphated to form coatings for which the quality standard generally reached is guaranteed.
Phosphating with addition of oxidizing agents has long been practiced in the automotive industry. By contrast, in the cold-forming field, several processes are operated on the iron side, i.e. with an increased content of iron(II) and no addition of oxygen-yielding accelerators.
The disadvantage of conventional phosphating baths lies in particular in the use of nitrites and/or chlorates as accelerators for the phosphating reaction. The nitrous gases formed where nitrites are used are known to have adverse effects on the usefulness of such baths.
2. Description of the Invention
The problem addressed by the present invention was to provide a nitrite-free and also chlorate-free phosphating process which, in particular, causes no so-called "fish eyes". Fish eyes are understood on the one hand to be buildups of zinc salts which are known in English as "white staining" and which are caused by the cleaning solution. On the other hand, crater-like openings known in English as "nubbing" are formed during phosphating. The systems normally used contain chlorate, bromate, nitrate, nitrite, peroxide and/or organic nitro compounds, such as 3-nitrobenzene sulfonate, as accelerators. Hitherto, 3-nitrobenzene sulfonate has generally been used together with chlorate and/or nitrite accelerators. In the presence of nitrate, however, the system normally used, 3-nitrobenzene sulfonic acid/chlorate, produces the above-mentioned fish eyes on electrolytically galvanized steel, so that phosphating normally has to be carried out in the absence of nitrate.